Fabry-perot spectral image measurement

ABSTRACT

A system for wide-range spectral measurement includes one or more broadband sources, an adjustable Fabry-Perot etalon, and a detector. The one or more broadband sources is to illuminate a sample, wherein the one or more broadband sources have a short broadband source coherence length. The adjustable Fabry-Perot etalon is to optically process the reflected light to extract spectral information with fine spectral resolution. The detector is to detect reflected light from the sample, wherein the reflected light is comprised of multiple narrow-band subsets of the illumination light having long coherence lengths and is optically processed using a plurality of settings for the adjustable Fabry-Perot etalon, and wherein the plurality of settings includes a separation of the Fabry-Perot etalon plates that is greater than the broadband source coherence length but that is less than the long coherence lengths.

CROSS REFERENCE TO OTHER APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/074,455 entitled MONOLITHIC TUNABLE IMAGING FABRY-PEROTINTERFEROMETER filed Nov. 3, 2014 which is incorporated herein byreference for all purposes.

BACKGROUND OF THE INVENTION

A producer or reseller of items (including ingredients and components ofsuch items)—for example a manufacturer, but also including other partiesin the entire supply and distribution chain such as a supplier, awholesaler, a distributor, a repackager, and a retailer—especially, butnot limited to, high-value items, faces counterfeiting of the item.Counterfeiting includes the substitution, dilution, addition or omissionof ingredients or components of the item compared to its intendedproduct specification, as well as misrepresentation or diversion of thepackaged item from its intended course of sale. This leads to loss ofpotential revenue as counterfeit items are sold in the place of the realitem. Also, there can be health or product related damages caused by notusing an authentic item as opposed to a counterfeit—for example, thecounterfeit can perform differently or not at all as compared to anauthentic item. This is particularly acute in industries that can affecthealth and safety such as industries involved with pharmaceuticals,nutritional supplements, medical devices, food and beverages,construction, transportation, and defense.

As international criminal organizations become more sophisticated,existing packaging security is proving inadequate. The complexity ofmany industry supply chains—for example, the supply chain of thepharmaceutical industry—lends itself to entry points for adulterated orcounterfeit product(s), often found in carefully counterfeited andhigh-quality packaging, and sometimes in authentic packaging that haseither been stolen or as part of a repackaging operation.

In complex product supply chains and markets with variable pricing,opportunities for arbitrage exist for unscrupulous parties tomisrepresent product pricing without any change to the underlyingproduct, and thus benefit monetarily, for example, as in returns, rebateor charge-back fraud. Monetary gain or loss to either side of atransaction may also result from errors in record-keeping.

In addition to counterfeiting or product misrepresentation, items thatappear physically identical or similar, for example certain nutritionalsupplements, may actually contain different ingredients or components,but because of similar appearance may be unintentionally packaged orlabeled incorrectly. Even if the items are otherwise identical, they mayhave different properties associated with the particular lot or batchconditions; for example, pharmaceuticals that otherwise appear identicalmay have different expiration dates and be incorrectly labeled due tofailures or limitations in quality assurance protocols to ascertain suchdifferences.

For product development and research, it may be beneficial at times tostudy and authenticate performance of items that appear identical butare made differently to learn whether or how those differences affect anend use. At times, it is important in such studies—for example inclinically masked (or ‘blind’) studies leading to pharmaceuticaldevelopment—to be able to confidently identify the underlying itemwithout revealing the true identity to study participants. In the caseof pharmaceutical development and clinical trials, item-level identityerror may be introduced, for example, at the contract researchorganization that repackages the various product formulations intomasked unit-doses. Much time, cost, and effort goes into statisticalsampling and chemical analyses to verify the true identity of theunit-doses that are ultimately administered.

In the effort to attain positive health outcomes in a morecost-effective and timely manner, healthcare providers need to focus onthe adherence to health regimens, not just the efficacy of specificdrugs. Understanding when, where and how often medicine is prescribed bya doctor, accurately and timely dispensed from a pharmacy, received by apatient, and consumed by the patient is helpful in understanding andverifying the effectiveness of the overall health regimen. Recording andcollecting the data for appropriate analysis and study while also beingable to confirm the underlying identity of the medicine at each stage isimportant to the reliability of the information collected.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are disclosed in the followingdetailed description and the accompanying drawings.

FIG. 1 is an image illustrating an embodiment of a tag.

FIG. 2 is a diagram illustrating an embodiment of tag measurementgeometry.

FIG. 3 is a diagram illustrating an embodiment of spectral dependenceassociated with geometry.

FIG. 4 is a diagram illustrating an embodiment of a Fabry-Perot etalon.

FIG. 5A is a graph illustrating an embodiment of the transmission of anetalon.

FIG. 5B is a graph illustrating an embodiment of the finesse as afunction of the reflectivity of the surfaces of the etalon.

FIG. 6 is a graph illustrating an embodiment of center wavelengthtransmitted through a Fabry-Perot etalon for different gaps between thesurfaces.

FIG. 7 is a diagram illustrating an embodiment of a system for relativespectral measurement.

FIG. 8 is a diagram illustrating an embodiment of a system for relativespectral measurement.

FIG. 9 is a diagram illustrating an embodiment of an integrated devicewith a Fabry-Perot etalon and an adjacent detector.

FIG. 10A is a diagram illustrating an embodiment of a step in producingthe integrated device.

FIG. 10B is a diagram illustrating an embodiment of a step in producingthe integrated device.

FIG. 11A is a diagram illustrating an embodiment of a step in producingthe integrating device.

FIG. 11B is a diagram illustrating an embodiment of a step in producingthe integrating device.

FIG. 12 is a diagram illustrating an embodiment of a step in producingthe integrating device.

FIG. 13 is a diagram illustrating an embodiment of an integrated devicewith a Fabry-Perot etalon and an adjacent detector.

FIG. 14 is a flow diagram illustrating an embodiment of a process formaking a relative spectral measurement.

DETAILED DESCRIPTION

The invention can be implemented in numerous ways, including as aprocess; an apparatus; a system; a composition of matter; a computerprogram product embodied on a computer readable storage medium; and/or aprocessor, such as a processor configured to execute instructions storedon and/or provided by a memory coupled to the processor. In thisspecification, these implementations, or any other form that theinvention may take, may be referred to as techniques. In general, theorder of the steps of disclosed processes may be altered within thescope of the invention. Unless stated otherwise, a component such as aprocessor or a memory described as being configured to perform a taskmay be implemented as a general component that is temporarily configuredto perform the task at a given time or a specific component that ismanufactured to perform the task. As used herein, the term ‘processor’refers to one or more devices, circuits, and/or processing coresconfigured to process data, such as computer program instructions.

A detailed description of one or more embodiments of the invention isprovided below along with accompanying figures that illustrate theprinciples of the invention. The invention is described in connectionwith such embodiments, but the invention is not limited to anyembodiment. The scope of the invention is limited only by the claims andthe invention encompasses numerous alternatives, modifications andequivalents. Numerous specific details are set forth in the followingdescription in order to provide a thorough understanding of theinvention. These details are provided for the purpose of example and theinvention may be practiced according to the claims without some or allof these specific details. For the purpose of clarity, technicalmaterial that is known in the technical fields related to the inventionhas not been described in detail so that the invention is notunnecessarily obscured.

A system for wide-range spectral measurement is disclosed The systemcomprises one or more broadband sources, an adjustable Fabry-Perotetalon, and a detector. The one or more broadband sources are toilluminate a sample, wherein the one or more broadband sources have ashort broadband source coherence length. The adjustable Fabry-Perotetalon is to optically process the reflected light to extract spectralinformation with fine spectral resolution. The detector is to detectreflected light from the sample, wherein the reflected light iscomprised of multiple narrow-band subsets of the illumination lighthaving long coherence lengths and is optically processed using aplurality of settings for the adjustable Fabry-Perot etalon. Theplurality of settings includes a separation of the Fabry-Perot etalonplates that is greater than the broadband source coherence length butthat is less than the coherence lengths of the reflected peaks.

In some embodiments, what is unique about the system setup is that oneor more wide band sources (this typically means short coherence length)are used but the tag decoding system requires very fine spectralresolution (which in a Fourier-type FPI means the etalon plates need tobe scanned to long distances). This is typically self-contradictory(i.e., if coherence is lost over large plate separations, theinterference patterns cannot be acquired and the system cannot operate).However, the target tags generate narrow reflection peaks whichtherefore have long coherence length and so the system as a whole isable to operate.

In some embodiments, the system for relative spectral measurement isspecifically configured to measure tags (e.g., rugate tags). In variousembodiments, tags comprise one of the following materials: silicon,silicon dioxide, silicon nitride, doped silicon, or any otherappropriate material. In some embodiments, tags are made of silica(deemed “generally recognized as safe”—or GRAS—by the FDA), renderingthem biologically inert and edible. Each barely visible tag contains acustom-manufactured spectral signature chosen so as to uniquely identifyor authenticate a particular product. Tags with a given spectralsignature are manufactured in quantities sufficient to enablecost-effective identification of commercial-scale product volumes. Thenumber of available spectral signature combinations range fromidentifying product manufacturer or brand, to product type or model, toindividual lot or batch numbers across multiple industries and markets.

In some embodiments, the unique optical signature of each tag can beread using an absolute or a relative spectral measurement device,apparatus, or system. In some embodiments, tags comprise the surface ofa silicon wafer that is etched to have a spectral code encoded by theetching. A thin layer from the surface of the etched wafer is removedand divided into small tags, and the resultant tags contain a complexporous nanostructure that is programmed during electrochemical synthesisto display a unique reflectivity spectrum. The tags are then oxidized bya high-temperature bake step to turn the crystalline, nanoporous silicontags into amorphous, nanoporous silica. This bake step stabilizes thenanoporous structure against further oxidation (thus stabilizing thespectral signature) and provides for the tags to be characterized as aGRAS excipient.

In some embodiments, the spectrum of one or more tags is measured in anabsolute or relative spectral measurement system, then verified againstother information as part of a database or located on a label orpackage. In some embodiments, the tags are used on their own actingsimply as labels for quality assurance or other purposes. Informationcapacity is based on the number of possible unique spectra, usingdifferent peak numbers, peak placements, peak rugate phases, and/or peakamplitudes as modulation parameters. The tags are passive, inconspicuousand can be attached to the outside of medicines or food products to beread, for example, through clear or translucent plastic blister packs,or mixed into medicines or food as a forensic excipient, to be read aspart of an investigation or inspection process by authorized security orquality assurance personnel.

In various embodiments, the tag properties comprise one or more of thefollowing:

-   -   Inconspicuous size range (≈50 to 100 micrometers) allows covert        or semi-covert use    -   Edible and biologically inert    -   High temperature resistance—melting point above 1000° C.    -   Passive—no energy input or output    -   Are used in or on a product, package, label, or security fiber    -   Are applied via sprays, coatings, varnishes, or as part of        laminate    -   Are integrated at a number of manufacturing stages    -   High level of security possible; can be scaled to suit specific        product needs    -   Are made so as to be self-authenticating and thereby have a        reduced cost and security risk as compared to systems with        online databases and maintenance

In some embodiments, the system for relative spectral measurementincludes a lens for collecting light from the sample with a good workingdistance and field of view (e.g., ˜10 mm diameter field of view, 2×objective lens with numeric aperture (NA) of ˜0.05-0.07, and workingdistance of ˜3-7 mm). In some embodiments, the objective lens will beoperated in a telecentric arrangement to ensure that the system capturestilted tags.

In some embodiments, lenses in the system for spectral measurement arepresent to separate the NA of the Fabry-Perot etalon from the NA of therest of the system. The system has a low NA at the Fabry-Perot etalon toavoid smearing the interferogram because light has traveled through thedevice at many different angles. In some embodiments, all the lenses arebroadband—there is no need for filtering the light with the exception ofthe Fabry-Perot etalon. In some embodiments the light reaching thedetector is bandpass filtered: either by sequentially illuminating withband limited sources, or by placing a series of bandpass filters betweenthe source and the tagged object, or by placing a set of filters betweenin the reflected path between the object and the detector or byutilizing a set of filters on the detector itself, or using acombination of the above. In various embodiments, the Fabry-Perot etalonis made of aluminum coated fused silica, glass or another dielectric, orany other appropriate material. In some embodiments, the outer surfacesof the Fabry-Perot etalon are anti-reflection coated. Scan range of theFabry-Perot etalon may be tuned from a smallest achievable gap beforethe plates stick together (e.g., ˜500 nm or less) to a gap at which acoherent signal is no longer present (e.g., ˜40 um). Depending on theFabry-Perot etalon finesse, signal processing for the system varies:

-   -   a. low finesse Fabry-Perot etalon—after background compensation        and optionally tag detection, take a windowed Fourier transform        of the interferogram to obtain a tag spectrum.    -   b. medium finesse Fabry-Perot etalon—after background        compensation and potentially tag detection, take a Fourier        transform; although the spectrum contains features, deal with        the presence of the features rather than try to deconvolve them.    -   c. high finesse Fabry-Perot etalon—after background compensation        and potentially localization of tag position, record the images        of transmitted light, determine the relative contribution of        interference orders by deconvolving with known RGB color filter        response as is known in the art, and continue until all        wavelength bands are covered.    -   d. Use a priori knowledge of the reflected tag spectra in order        to detect the spectral signature at ultra-low signal-to-noise        regimes.

In some embodiments, because of the telecentric position of theFabry-Perot etalon, each spot on the sample is imaged to a single spoton the Fabry-Perot etalon. The Fabry-Perot etalon is not required tomaintain the same gap everywhere because only a relative spectrummeasurement is required. In some embodiments, absolute parallelism isnot required, but some reference is required for the system—for example,a laser, to calibrate. In some embodiments, we may be able to workwithout a reference (e.g., by applying a known spectral feature in thetag, serving as an anchor), thus simplifying and cost-reducing thesystem. Each object/tag in the image can be processed independently,without the need for normalizing for differences in plate separation.This is different from most imaging interferometer applications, andallows relaxed specifications on optical flatness and coatinguniformity, and even coplanarity of the Fabry-Perot etalon.

FIG. 1 is an image illustrating an embodiment of a tag. In the exampleshown, tag 100 is a 50 um to 100 um sized irregularly shaped tag. Thetags are 20 um thick. These tags are imaged using an Ocean OpticsUSB2000+ spectrometer with broadband illumination from a halogen source

FIG. 2 is a diagram illustrating an embodiment of tag measurementgeometry. In the example shown, tag 202 is partially embedded insubstrate 200 (e.g., at an angle to the surface of the substrate).Illumination beam (e.g., beam 205) is incident within angular coneoutlined by 207 and 208. Collection aperture is different fromillumination beam and is outlined with 203 and 204. In this case, thelarger collection aperture as compared to the illumination beam enablesthe collection of reflected light from tilted tags. In some embodiments,beam 206 is reflected beam from incident beam 205.

FIG. 3 is a diagram illustrating an embodiment of spectral dependenceassociated with geometry. In the example shown, tag 352 surface normalis angle θ with respect to incident light 355. A reflected spectral peaklocation is a function of the optical path length of the beam within thetag. The path is a function of the angle between the light ray and thesurface of the tag and is proportional to d/cos(θ). Thus, the spectrumfrom a tilted tag is shifted. In addition, the peaks broaden and becomelower (perhaps due to scattering within the tag). In some embodiments,beam 356 is reflected beam from incident beam 355.

In some embodiments, the reflections of multiple tags at differentangles will broaden the reflection peaks. In some embodiments,variations in tags also will broaden the reflection peaks.

In some embodiments, the tags are specular reflectors and not diffusereflectors. Therefore, the tags can either be illuminated at a largeangle orthogonal to the object surface, or the tags can be combinedusing a diffuser and lens to form a combined multispectral high NA beamorthogonal to the surface

FIG. 4 is a diagram illustrating an embodiment of a Fabry-Perot etalon.A related device is the Fabry-Perot Interferometer or etalon. The heartof the Fabry-Perot etalon (e.g., etalon 452) is a pair of partiallyreflective surfaces (e.g., surface 464 and surface 466) spaced hundredsof nanometers to centimeters apart (e.g., d). Light is incident at anangle θ to the normal of surface 464. The first reflection 456 is R₀.Within etalon 452 light travels at angle α to the normal of surface 466(see also close up). The varying transmission function of the etalon iscaused by interference of the multiple reflections of light between thetwo reflecting surfaces. Producing beams transmitted 460 (T₁, T₂, T₃,etc.) and reflected 462 (R₁, R₂, R₃, etc.). Constructive interferenceoccurs if the transmitted beams are in phase, and this corresponds to ahigh-transmission peak of the etalon. If the transmitted beams areout-of-phase, destructive interference occurs and this corresponds to atransmission minimum. Whether the multiple reflected beams are in phaseor not depends on the wavelength (λ) of the light, the angle the lighttravels through the etalon (α), and the local thickness of the etalon(d). In the equations below, the plates are separated by a space with arefractive index n (e.g., for air n=1) and the propagation of light intothe plates is negligible or independent of wavelength. The phasedifference between each successive transmitted pair (e.g., T₂−T₁) isgiven by δ:δ(=(2π/λ)2nd cos αIf both surfaces have a reflectance R, the transmittance function of theetalon is given by:

${T_{e} = {\frac{\left( {1 - R} \right)^{2}}{1 + R^{2} - {2R\;\cos\;\delta}} = \frac{1}{1 + {F\;{\sin^{2}\left( {\delta/2} \right)}}}}},$where the coefficient of finesse (F) is

$F = \frac{4R}{\left( {1 - R} \right)^{2}}$

FIG. 5A is a graph illustrating an embodiment of the transmission of anetalon. In the example shown, maximum transmission of the etalon(T_(e)=1) occurs when the optical path length difference, 2nd cos α,between each transmitted beam is an integer multiple of the wavelength(λ). In the absence of absorption, the reflectance of the etalon R_(e)is the complement of the transmittance, such that T_(e)+R_(e)=1. Themaximum reflectivity is given by:

$R_{\max} = {{1 - \frac{1}{1 + F}} = \frac{4R}{\left( {1 + R} \right)^{2}}}$and this occurs when the path-length difference is equal to half an oddmultiple of the wavelength. A high-finesse etalon (F=10) shows sharperpeaks and lower transmission minima than a low-finesse etalon (F=2). Thewavelength separation between adjacent transmission peaks is called thefree spectral range (FSR) of the etalon, Δλ, and is given by:Δλ=λ² ₀/2nd cos α+λ₀)where λ₀ is the central wavelength of the nearest transmission peak. TheFSR is related to the full-width half-maximum, δλ, of any onetransmission band by a quantity known as the finesse:

$\mathcal{F} = {\frac{\Delta\;\lambda}{\delta\;\lambda} = \frac{\pi}{2\;{\arcsin\left( {1/\sqrt{F}} \right)}}}$

A Fabry-Perot etalon is able to adjust the distance d between thereflective surfaces in order to change the wavelengths at whichtransmission peaks occur in the etalon. Due to the angular dependence ofthe transmission, the peaks can also be shifted by rotating the etalonwith respect to the beam or if the beam enters the etalon at an angle.In this case, the transmitted wavelengths will shift by the cosine ofits angle with the plates. This result is important because it meansthat if light is not perfectly collimated as it enters the etalon, thetransmission peaks will be broadened and spectral resolution will bedecreased. This angular dependence has different effects depending onthe optical configuration in which the etalon is used. In a telecentriccase, at each location on the entrance plane to the etalon, rays areentering at a different angle. Therefore, the spectral response at eachlocation will be different, although the effect of plate flatness ornon-coplanarity will be reduced. For a given plate separation, multiplewavelengths will be transmitted through the device. For a given Finesse,as the wavelength resolution increases, the FSR decreases.

FIG. 5B is a graph illustrating an embodiment of the finesse as afunction of the reflectivity of the surfaces of the etalon. In theexample shown, high finesse factors correspond to high reflectivity ofthe etalon surfaces.

FIG. 6 is a graph illustrating an embodiment of center wavelengthtransmitted through a Fabry-Perot etalon for different gaps between thesurfaces. For a given plate separation, multiple wavelengths will betransmitted through the device. In some embodiments inserting a bandpassfilter ensures that wavelengths from only one interference pattern enterthe Fabry-Perot etalon. By using different bandpass filters, differentorders through the Fabry-Perot etalon can be identified. For example,using a multispectral image sensor at the exit of the Fabry-Perotetalon, such that by looking at the relative amplitude of light ondifferent pixels, one can deduce which harmonics was transmitted throughthe Fabry-Perot etalon. In some embodiments, two Fabry-Perot etalons areused in series to isolate one order—the first having a low finesse andacting as a bandpass filter to select a limited band of light, narrowerthan the FSR of the second Fabry-Perot etalon, which has a high Finesse(low FWHM with narrow FSR). Note that implementations where the deviceis used as a bandpass filter are very lossy in terms of their energyutilization, because only a tiny fraction of the incoming light istransmitted through the device and is used for generating the spectrum.

For a typical Fourier-Transform Fabry-Perot Interferometer (FT FPI), thetwo parallel partially-reflective mirrors of the Fabry-Perot etalon areutilized in a different way. FT-FPI's take 2D images of the (in theideal case) monochromatic light exiting the Fabry-Perot etalon for eachinter-plate separation, to generate a hyperspectral cube. In otherwords, an inter-plate separation is set (d), and a two-dimensional imageof an illuminated object is taken that gives frequency information ofthe light reflected from the object. To obtain an accuraterepresentation of the entirety of the spectrum a zero separation (e.g.,d=0) image must be included in the interferogram. However this is notpractical given both photon penetration depth into the glass coatingmaterial and the tendency of 2 flat plates to adhere if placed in closeproximity. Multiple methods have been developed in order to get aroundthis problem, including inferring the small gap portion of theinterferogram from the Fourier transform of the accessible portion ofthe interferogram along with a priori knowledge of the spectrum to bemeasured. For producing a device with a narrowly defined function suchas measuring tags, more a priori knowledge is available and tis is lessof a problem.

Also, ideally for the FT-FPI the light incident has long coherencelength for all the wavelengths that are desired to be measured. Theoperation of the FT-FPI is based on the coherence of the reflectedlight. In other words, as wave fronts pass through multiple reflections,they must remain in-phase in order for them to consistently interfere.This is less an issue for laser light which is highly coherent, but whena broadband or white light source (e.g., from an LED) passes multipletimes through plates which are placed far apart, this can impose acritical constraint on the system. For example, assume a maximumseparation of 1 mm between the plates and a reflectivity such that theamplitude of reflected light is still significant after 10 reflections,this translates to a total coherence length requirement of 10 mm. But atypical coherence length of a broadband or white light source is a fewhundred nanometers to a few microns. This can limit the number ofinterference orders which are used in the transform and therefore thequality of the resultant spectrum that is measured by the FT-FPI.

The characteristics of the tags being measured enable a specializedvariant of a Fourier-Transform Fabry-Perot Interferometer. The systemfor relative spectral measurement differs from FT-FPI as follows:

-   -   No bandpass filter needed at entrance (e.g., no bandpass filter        between the sample and the Fabry-Perot etalon). The spectral        location of the tag (e.g., rugate peaks) assures sufficiently        low spectral content in longer wavelengths. Thus, no bandgap or        low-pass filter is required at the entry to prevent ambiguity in        the resultant spectrum.    -   Large distance d range can be scanned with incoherent wide band        source (e.g. 400-900 nm). The tag-generated peaks are quite        narrow (5 nm-20 nm). Since coherence length increases as the        inverse of linewidth, the tag reflectance characteristics in        effect extend the usable range of the FT-FPI. For example, at        850 nm, a conservative estimate of the coherence length of the        rugate peaks is 36 μm with a 20 nm (widest possible) peak FWHM.        Therefore, the device can be operated with long separation        ranges to yield good Fourier Transform results. In contrast, if        the peak was not present and we were trying to image an object        with a 200 nm spectral content, the coherence length would have        been only 3.6 μm and we would not be able to generate a reliable        Fourier Transform. A typical FT-FPI would not be able to        function properly across the required spectral range unless        objects such as the tags significantly reduce the linewidth of        the observed object.    -   The manufacturing tolerances can be greatly relaxed (e.g., tens        of nanometers). Specifically, because the spectral response is a        function of the separation between the surfaces, non-flatness or        roughness of either surface results in spectral broadening        (e.g., localized averaging of the phases) as well as a spectral        response which is a function of position on the device—each        point in the image cannot be interpreted as corresponding to the        same wavelength (or in the case of curved surface of an FT-FPI a        smoothly varying wavelength). However, because only relative        spectral measurements are being measured, each image point can        be interpreted separately not absolutely and therefore        calibrated individually. As an example, for the relative        measurement system each element samples a unique area element of        the object, on the order of or smaller than a single tag (e.g.,        a 100 mm² region is imaged using a 10 MPixel sensor, then a 100        μm diameter tag will span 0.1²/10²×10⁷=1,000 pixel). Each such        area element will have a slightly different spectral response,        but the FWHM will still be narrow, especially if plate        reflectivity is kept low such that each beam makes only one        round trip before it is significantly attenuated.    -   No requirement for zero separation of plates. The tag signal can        be decoded without knowing absolute wavelength: As discussed        above, prior work on FT-FPI required OPD to be scanned from zero        separation to a sufficiently large separation (such that many        interference orders are recorded) in order to perform a Fourier        Transform. This imposes difficult constraints on the design of        the plates, to prevent them from sticking together. We perform a        Finite Fourier Transform starting from a finite separation and        miss some lower interference orders. Thus, strictly speaking,        the system comprises not a Fourier-Transform Fabry-Perot        interferometer but rather a Finite-Fourier-Transform Fabry-Perot        interferometer, and delivers not an absolute spectrum but rather        a relative spectrum which varies by position across the detector        array. While unacceptable from most hyperspectral imaging        applications, this is absolutely acceptable for decoding tags        and enables a lower complexity and lower cost device.

FIG. 7 is a diagram illustrating an embodiment of a system for relativespectral measurement. In the example shown, source 700 providesbroadband illumination to sample sitting on or in substrate 714. Forexample, source 700 comprises a white light emitting diode, a tungstensource, an incandescent source, or any other appropriate source. Lightfrom source 700 propagates along path 702 and is collimated using lens704. Light propagates along path 706 and at least a portion is reflectedby beam splitter 708 through objective 710, travels on path 712, and isfocused on sample on substrate 714. Reflected light from sample onsample substrate 714 propagates along path 716. Numerical aperture ofincident beam is different from the numerical aperture of the reflectedbeam (e.g., NA of incident beam is smaller than NA of reflected beam).

In the example shown, reflected light from sample is collimated andpropagates along 718 with at least a portion of the beam transmittingthrough beam splitter 708. The reflected light is focused using lens 720to focus on Fabry-Perot etalon 724 on path 722. For example, thereflected light of the sample is imaged on to the Fabry-Perot etalon(e.g., Fabry-Perot etalon 724). The transmitted light throughFabry-Perot etalon 724 propagates along path 726 to lens 728 and lens732 so that the transmitted light propagates along path 730 and path734. Transmitted light is focused on detector 736. For example, thefiltered reflected light from the sample is imaged onto the detector(e.g., detector 736). Detector 736, Fabry-Perot etalon 724, and sampleon sample substrate 714 are each optically at the same point (e.g.,telecentric). In some embodiments, detector 736 and Fabry-Perot etalon724 are separated by imaging optics (e.g., one or more lenses).

FIG. 8 is a diagram illustrating an embodiment of a system for relativespectral measurement. In the example shown, source 800 providesbroadband illumination to sample sitting on or in substrate 814. Forexample, source 800 comprises a white light emitting diode, a tungstensource, an incandescent source, or any other appropriate source. Lightfrom source 800 propagates along path 802 and is collimated using lens804. Light propagates along path 806 and at least a portion is reflectedby beam splitter 808 through objective 810, travels on path 812, and isfocused on sample on substrate 814. Reflected light from sample onsample substrate 814 propagates along path 816. Numerical aperture ofincident beam is different from the numerical aperture of the reflectedbeam (e.g., NA of incident beam is smaller than NA of reflected beam).

In the example shown, reflected light from sample is collimated andpropagates along 818 with at least a portion of the beam transmittingthrough beam splitter 808. The reflected light is focused using lens 820to focus on Fabry-Perot etalon 824 on path 822. The transmitted lightthrough Fabry-Perot etalon 824 propagates directly to detector 836.Detector 836, Fabry-Perot etalon 824, and sample on sample substrate 814are each optically close to the same point.

In some embodiments, Fabry-Perot etalon 824 and detector 836 arecombined in an integrated device. The integrated device addresses thefollowing problems:

-   -   Co-location of the Fabry-Perot cavity with the detector on the        focal plane to maximize spatial and spectral performance    -   Reduction of system complexity, robustness, and cost by reducing        the number of optical elements.

In some embodiments, the integrated device integrates the image sensor(e.g., a complementary metal-oxide-semiconductor (CMOS) Image Sensor,charge coupled device (CCD), or any other array sensor) as theback-reflector of a Fabry-Perot etalon. Typically, Fabry-Perot etalonsurfaces or plates need to have the following performance:

-   -   Known and preferably controlled reflectivity (e.g., by applying        a specific coating)    -   Flat, typically less than the shortest wavelength of light to be        passed through the FPI.

In some embodiments, the front surface of CMOS Image Sensors (CIS) isnot used. Specifically, because the front surface of the CIS contains astack of metal lines separated by dielectrics with additional structureson top, the top surface typically has local non-planarity in the micronrange, and even polishing cannot achieve tens of nanometers peak tovalley flatness. Furthermore, for this reason and because of patterningof the top surface, the reflectivity varies across the top surface ofthe CIS die.

In some embodiments, backside-illuminated (BSI) CIS technologies havethe back surface of the CIS wafer that is backgrinded, and photonsimpinge on the back surface of the die, are absorbed in the silicon andcollected and processed as before. In this scheme, the side exposed tothe light is flat (e.g., unpatterned and typically polished silicon).This side can be coated by anti-reflective coating to reduce or controlits reflectivity. In some embodiments, that surface is polished to reach“optical” quality of tens of nanometers peak to valley across a die.This surface can be used as the back plate of a Fabry-Perot etalon.

In various embodiments, the relative spectral measurement devicecomprises a fixed and/or a tunable Fabry-Perot etalon, both integratedwith the array detector such that it forms a monolithic unit.

FIG. 9 is a diagram illustrating an embodiment of an integrated devicewith a Fabry-Perot etalon and an adjacent detector. In some embodiments,the integrated device of FIG. 9 is used to implement Fabry-Perot etalon824 and detector 836 of FIG. 8. In the example shown, glass 900 has thinmetal coating 902 separated from anti-reflective coating 906 by spacer903 and spacer 904. In various embodiments, spacer 903 and spacer 904comprise a plurality of static spacers, active spacers,Micro-Electro-Mechanical Systems (MEMS) movers or stagers, piezoelectricmovers or stages, or any other appropriate spacers between the frontreflector and the CMOS detector die. In some embodiments, three stagesare attached so that coplanarity is achievable between the two surfacesof the Fabry-Perot etalon by adjusting the stages. Anti-reflectivecoating 906 is coupled to silicon substrate 908 with photodiodes 907.Silicon substrate 908 with photodiodes 907 is coupled with silicondioxide inter-metal dielectric 918 with metal wires embedded (e.g.,metal wires 910, metal wires 912 and metal wires 914). The top comprisesfront side 916 of the integrated device. Illumination is from thebottom.

In some embodiments, the back surface of the BSI wafer is polished toachieve desired flatness.

In some embodiments, an etch stop layer is deposited on the back surfaceof the wafer such that a uniform-thickness layer is formed—for example,using Atomic Layer Deposition.

In some embodiments, the anti-reflective coating on the back side of thesilicon die is present. In some embodiments, the anti-reflective coatingon the back side of the silicon die is not present. In some embodiments,there is a thin metal coating on the back side of the silicon die and noanti-reflective coating. In the event that no coating is used, thereflectivity of the silicon surface as a function of wavelength must betaken into account when calculating the spectral response of themonolithic device. In some embodiments, the spectral response of theanti-reflective coating is designed—for example, such that itcomplements the reflectivity of the silicon surface resulting in aspectrally flat response. In some embodiments, a thin metal coating isapplied on the silicon surface such that the target reflectivity isreached, while a sufficiently high portion of the light (e.g., 20%reflectivity and 80% transmissivity) is transmitted to the photodiodesin the silicon die.

In some embodiments, an important feature of the glass—BSI CMOS—spacerand the glass—BSI CMOS with a tunable MEMS interposer configurations isthat they are amenable to wafer-level integration, and is thus very costeffective.

FIG. 10A is a diagram illustrating an embodiment of a step in producingthe integrated device. In the example shown, a CIS wafer is fabricated.Silicon substrate 1008 with photodiodes 1007 is coupled with silicondioxide inter-metal dielectric 1018 with metal wires embedded (e.g.,metal wires 1010, metal wires 1012 and metal wires 1014). The topcomprises front side 1016 of the integrated device.

FIG. 10B is a diagram illustrating an embodiment of a step in producingthe integrated device. In the example shown, handle wafer 1070 isattached to front surface 1066, and silicon substrate 1058 is etched tothe desired thickness and subsequently polished to the desired flatness.Silicon substrate 1058 with photodiodes 1057 is coupled with silicondioxide inter-metal dielectric 1068 with metal wires embedded (e.g.,metal wires 1060, metal wires 1062 and metal wires 1064). The topcomprises front side 1016 of the integrated device.

FIG. 11A is a diagram illustrating an embodiment of a step in producingthe integrating device. In the example shown, glass or quartz wafer 1100(e.g., nominal thickness or previously backgrinded and optionallyattached to a handle wafer from the bottom) is coated by a metal film1102 (e.g., metal coating).

FIG. 11B is a diagram illustrating an embodiment of a step in producingthe integrating device. In the example shown, glass or quartz wafer 1150(e.g., nominal thickness or previously backgrinded and optionallyattached to a handle wafer from the bottom) is coated by a metal film1152 (e.g., metal coating). A spacer (e.g., spacer 1153 and spacer 1154)is either deposited and patterned (e.g., using polyimide) or iswafer-bonded (e.g., similar to bonding of interposer wafers as is knownin the industry), or is etched from the original glass wafer prior tothe thin metal film coating. In all cases, the spacer is patterned suchthat the wafers' separation is similar to the dimension of the dies onthe CIS wafer. More specifically, in the case of a polyimide spacer, theprocess flow is:

-   -   Spin coat polyimide and cure    -   Optionally polish the polyimide    -   Photolithographically etch the non-spacer regions

FIG. 12 is a diagram illustrating an embodiment of a step in producingthe integrating device. In the example shown, the glass wafer and CISwafer are aligned and bonded—for example, by heating the polyimide layerwhile concurrently co-planarizing the wafers and cooling. Glass 1200 hasthin metal coating 1202 separated from silicon substrate 1208 by spacer1203 and spacer 1204. In various embodiments, spacer 1203 and spacer1204 comprise static spacers, active spacers, Micro-Electro-MechanicalSystems (MEMS), piezoelectric movers, or any other appropriate spacers.Silicon substrate 1208 includes photodiodes 1207. Silicon substrate 1208with photodiodes 1207 is coupled with silicon dioxide inter-metaldielectric 1218 with metal wires embedded (e.g., metal wires 1210, metalwires 1212 and metal wires 1214). Handle wafer 1220 is attached to frontsurface 1216. In some embodiments, the handle wafer is de-bonded.

In some embodiments, the CIS is electrically and mechanically bonded toan electrical substrate as is done with CIS camera modules.

FIG. 13 is a diagram illustrating an embodiment of an integrated devicewith a Fabry-Perot etalon and an adjacent detector. In some embodiments,the integrated device of FIG. 13 is used to implement Fabry-Perot etalon824 and detector 836 of FIG. 8. In the example shown, glass 1300 hasthin metal coating 1302 separated from anti-reflective coating 1306 byspacer 1303 and spacer 1304 and spectral filters 1305. In variousembodiments, spacer 1303 and spacer 1304 comprise static spacers, activespacers, Micro-Electro-Mechanical Systems (MEMS), piezoelectric movers,or any other appropriate spacers. Anti-reflective coating 1306 iscoupled to silicon substrate 1308 with photodiodes 1307. Siliconsubstrate 1308 with photodiodes 1307 is coupled with silicon dioxideinter-metal dielectric 1318 with metal wires embedded (e.g., metal wires1310, metal wires 1312 and metal wires 1314). The top comprises frontside 1316 of the integrated device.

In some embodiments, an array of spectral (e.g., color) filters isdeposited on the backside of the CIS wafer, such that the spectralresponse of different pixels changes. In some embodiments, the filterarray is deposited between the Silicon wafer and the anti-reflection ormetal-film layer. In some embodiments, the pattern density of the colorfilter array is higher than the spatial density of information to becollected from the object (i.e., the objects is spatially oversampled).In some embodiments, harmonics, which are an artifact of Fabry-Perotetalons, are resolved by oversampling the image on multiple pixels, eachwith a different filter and therefore a different spectral response, andthus the usable spectral range of the device is extended beyond its FreeSpectral Range. In some embodiments, the color filter array is depositedon the glass plate and then passivated. In some embodiments, the colorfilter array is deposited on the flat, back surface of the wafer and noton the front surface. In some embodiments, a planarization layer isdeposited on top of the color filter array—for example, in the form oflow-temperature Chemical Vapor Deposition (CVD) or Physical VaporDeposition (PVD) of SiO₂ and then planarized by chemical and/ormechanical polishing.

In some embodiments, Fabry-Perot interferometers have a limited spectralrange, and so for these interferometers, the system includes two etalonsin series—one in low finesse mode to select a band of wavelengths, andanother in high finesse mode. In some embodiments, the BSI-CMOS etalonis the second one (e.g., the high finesse etalon) in this configuration.

In some embodiments, one or more MEMS wafers have dies of the same sizeas the BSI CMOS dies and comprises linear actuators (per die), which canadjust the spacing between the glass wafer and the BSI CMOS wafer. TheMEMS wafer or a stack of MEMS wafers is placed in place of or inaddition to the patterned spacer (e.g., as described above). Electricalcontacts to enable adjustment of said spacing is achieved using eitherby direct access to the MEMS device (after the stack is diced) or byother means, such as but not limited to by electrical contact tothrough-silicon-vias (TSVs) on the CMOS BSI wafer.

In some embodiments, metallic capacitive elements are deposited on thesurface of the glass plates of a Fabry-Perot etalon, and these arerouted to a signal generator and measurement circuitry to monitorcapacitance and therefore inter-plate separation. In variousembodiments, a capacitive plate is deposited on the front glass plateand grounded, and a second capacitive plate is deposited on the CMOSfront surface (just below the glass passivation) and connected—forexample, using TSV's to capacitive measurement circuitry on the CMOSdie, or connected using TSV's to a pad on the top surface of the diewhich is electrically connected to an off-chip capacitive measurementcircuit, or in any other appropriate manner.

In various embodiments, either the glass wafer or the BSI CMOS wafer isbonded or otherwise coated with a patterned layer or layers (e.g.,piezo-electric transducer), which, upon actuation with an electric fieldcan change the separation between the glass and BSI wafers.

In some embodiments, a process flow for fabricating the device is asfollows:

-   -   a CMOS BSI wafer is fabricated;    -   the CMOS BSI wafer is temporarily bonded on the front side to a        carrier wafer;    -   the bonded CMOS BSI wafer is backgrinded such that the desired        performance of the image sensor is achieved—for example,        sufficiently thin to reduce cross-talk but not so thin as to        reduce cross-talk or electrical performance.    -   through-Silicon-Vias (TSV) (e.g., using a “Via-Last” process)        are formed on the periphery of each BSI CMOS die;    -   a piezoelectric transducer (e.g., PZT) pattern is formed or        transferred on the CMOS BSI wafer such that the pattern is        repeated for all dies, that the center of the die is free of the        PZT, and that thickness of the piezoelectric material is on the        order of a few to a few tens of microns. The piezoelectric        material is electrically bonded to the TSVs on the CMOS BSI        wafer. Alternately, this patterned piezoelectric transducer        pattern may be formed or transferred on a second, glass or        quartz wafer;    -   the glass and BSI CMOS wafer are bonded and diced to form        Fabry-Perot etalon.

In some embodiments, the wafer spacer described above and the bonding ofthe two wafers is achieved using techniques. Several examples are asfollows:

-   -   adhesive wafer bonding using photosensitive polymer layer. A        polymer layer is spun on one of the wafers (preferably the glass        wafer, photolithographically patterned, soft baked, aligned to        the CMOS wafer, and bonded);    -   UV curable adhesives (UV curing via the glass wafer) or laser        curable adhesives;    -   wafer-level camera module spacer attach process.

FIG. 14 is a flow diagram illustrating an embodiment of a process formaking a relative spectral measurement. In the example shown, in 1400 abroadband source is provided to illuminate a sample. For example, theone or more broadband sources provide illumination that is used toilluminate the sample and the one or more broadband sources have a shortbroadband source coherence length. In 1402, a lens is provided tocollect reflected light from the sample. In 1404, an adjustableFabry-Perot etalon is provided to filter the reflected light. Forexample, the adjustable Fabry-Perot etalon is to optically process thereflected light to extract spectral information with fine spectralresolution. In 1406, a detector is provided to detect reflected lightfrom the sample, wherein the reflected light is comprised of multiplenarrow-band subsets of the illumination light having long coherencelengths and is optically processed using a plurality of settings for theadjustable Fabry-Perot etalon, and wherein the plurality of settingsincludes a separation of the Fabry-Perot etalon plates that is greaterthan the broadband source coherence length but that is less than thelong coherence lengths. In some embodiments, the detector detectsreflected light associated with a plurality of settings for theadjustable Fabry-Perot etalon, where the plurality of settings iswithout a setting of zero separation of the Fabry-Perot etalon plates.

Although the foregoing embodiments have been described in some detailfor purposes of clarity of understanding, the invention is not limitedto the details provided. There are many alternative ways of implementingthe invention. The disclosed embodiments are illustrative and notrestrictive.

What is claimed is:
 1. A system for wide-range spectral measurement,comprising: one or more broadband sources to illuminate a rugate tag,wherein the one or more broadband sources have a short broadband sourcecoherence length, wherein light from the one or more broadband sourcesis reflected by a beam splitter toward a lens to illuminate the rugatetag, wherein the light illuminates the rugate tag at a first angularcone, wherein the lens to collect reflected light from the rugate tag ata second angular cone, wherein the second angular cone is greater thanthe first angular cone; an adjustable Fabry-Perot etalon to opticallyprocess the reflected light to extract spectral information with finespectral resolution; a detector to detect reflected light from therugate tag, wherein the reflected light is comprised of multiplenarrow-band subsets of the illumination light having long coherencelengths and is optically processed using a plurality of settings for theadjustable Fabry-Perot etalon, and wherein the plurality of settingsincludes a separation of the Fabry-Perot etalon plates that is greaterthan the broadband source coherence length but that is less than thelong coherence lengths; and a processor configured to determine if thedetected reflected light from the rugate tag displays a predeterminedreflectivity spectrum associated with the rugate tag.
 2. A system as inclaim 1, wherein the reflected light is imaged on to the adjustableFabry-Perot etalon.
 3. A system as in claim 1, wherein the adjustableFabry-Perot etalon is set at a first separation distance to measure aresponse at a first wavelength.
 4. A system as in claim 3, wherein theadjustable Fabry-Perot etalon is set at a second separation distance tomeasure a response at a second wavelength.
 5. A system as in claim 1,wherein the optically processed reflected light is imaged on to thedetector.
 6. A system as in claim 1, wherein the adjustable Fabry-Perotetalon is separated from the detector by imaging optics.
 7. A system asin claim 6, wherein the imaging optics comprise one or more lenses.
 8. Asystem as in claim 1, wherein the adjustable Fabry-Perot etalon and thedetector are combined in an integrated device.
 9. A system as in claim8, wherein the integrated device comprises a backside imaging sensor.10. A system as in claim 8, wherein the integrated device comprises aFabry-Perot etalon.
 11. A system as in claim 8, wherein the integrateddevice comprises one or more movers.
 12. A system as in claim 8, whereina mover of the one or more movers comprises a MEMS mover.
 13. A systemas in claim 8, wherein a mover of the one or more movers comprises apiezoelectric mover.
 14. A system as in claim 1, wherein the rugate tag,the Fabry-Perot etalon, and the detector are telecentric.
 15. A systemas in claim 1, wherein the rugate tag contains a complex porousnanostructure that is programmed during electrochemical synthesis todisplay the predetermined reflexivity spectrum.
 16. A method of relativespectral measurement, comprising: providing one or more broadbandsources to illuminate a rugate tag, wherein the one or more broadbandsources have a short broadband source coherence length, wherein lightfrom the one or more broadband sources is reflected by a beam splittertoward a lens to illuminate the rugate tag, wherein the lightilluminates the rugate tag at a first angular cone, wherein the lens tocollect reflected light from the rugate tag at a second angular cone,wherein the second angular cone is greater than the first angular cone;providing an adjustable Fabry-Perot etalon to optically process thereflected light to extract spectral information with fine spectralresolution; providing a detector to detect reflected light from therugate tag, wherein the reflected light is comprised of multiplenarrow-band subsets of the illumination light having long coherencelengths and is optically processed using a plurality of settings for theadjustable Fabry-Perot etalon, and wherein the plurality of settingsincludes a separation of the Fabry-Perot etalon plates that is greaterthan the broadband source coherence length but that is less than thelong coherence lengths; and determining, using a processor, whether thedetected reflected light from the rugate tag displays a predeterminedreflectivity spectrum associated with the rugate tag.